Correct positioning of surgical instruments and implants, used in a surgical procedure, with respect to the patient's anatomy is often a critical factor in achieving a successful outcome. In certain orthopaedic implant procedures, such as total hip replacement (THR), total knee arthroplasty (TKA) and total shoulder replacement (TSR), the optimal orientation of the surgical implant enhances the initial function and the long term operability of the implant.
With respect to THR, proper implantation of the acetabular component (cup) in total hip replacement (THR) is challenging. The hip is a ball and socket joint. In a normal hip, the femoral head is generally circular and rotates within the acetabulum which is also generally circular in shape. Ideally, the load transfer of body weight across the hip joint is distributed across the surface area of the femoral head and acetabulum. By distributing the loads across a maximum surface area of the femoral head and acetabulum, lower stresses result in the joint itself.
In a diseased hip, the ball and socket may be malformed and this can result in an uneven distribution of load. In the event of a deformed femoral head, load is transferred from the femoral head to the acetabulum along the periphery of the femoral head to the periphery of the acetabulum. As will be appreciated, this results in a transfer of load (imported by the body weight and muscle forces) to a surface area of much reduced size causing a high load per unit area value. This increased load per unit area causes damage to the joint by damaging the articular cartilage which over time can wear out.
From a biomechanical standpoint, successful hip function depends on a number of factors including the alignment of an acetabular cup. Accurate orienting of the cup is an important variable in reducing the risk of dislocation, bearing impingement, wear and edge loading, revisions, and long-term survivorship of the THR. Proper cup orientation can be determined by considering the appropriate abduction angle and anteversion of the component.
Various techniques involving bony landmarks or intra-operative jigs have been developed to allow the operating surgeon to produce accurate and reproducible cup placements. Alignment tools (guides) typically references the surgical table on which the patient rests. Conventionally, it is assumed that the patient's pelvis is parallel to the table, and the surgical table is parallel to the floor. Based on the proceeding assumptions, the ordinary orientation for most patients for the acetabular cup is between 40° and 45° of inclination and around 20° anteversion.
However, these techniques are subject to inaccuracies due to the variability of the patient's pelvic position on the operating table, degenerative lumbosacral disease, pelvic tilt, and the presence of osteophytes which makes bony landmarks harder to identify. For this reason, newer instruments utilizing computer assisted navigation or haptic robots have become more popular. However, these instruments are very expensive, require additional components (pins and rays) and have a long learning curve and thus are not entirely desirable for these reasons. In addition, as described below, a number of patient-specific guides have been developed by using the rim of the acetabulum to fit the patient-specific guide; however, the rim of the acetabulum is difficult to access as a result of it being covered in part by soft tissue and consequently, these guides suffer from this deficiency and others.
There is thus a need to overcome these deficiencies and provide an improved method(s) and instrument(s) for orienting the acetabular component (cup) during and as part of a total hip replacement procedure.